1. Field of the Invention
The present invention relates to an optical unit used in a projection optical system in a semiconductor exposure unit, an image-pickup device such as a video camera or a still camera, or other optical device.
2. Description of the Related Art
The method of pressing down a lens by means of a ring-shaped member known as a junk ring, as shown in FIG. 7, is conventionally used commonly for the purpose of holding a lens, an optical element, in a lens enclosure of an optical device.
In FIG. 7, a portion engaging with the outer periphery of a lens 51 and a supporting section 53a supporting the periphery of the optical surface of the lens 51 are formed along the inside diameter of a lens holding member 53 fixed within the lens enclosure 52. The optical surface on the opposite side of the lens 51 set on the lens holding member 53 is pressed down by means of a ring-shaped junk ring 54 screwed into a threaded portion formed on the inner periphery of the lens holding member 53.
The lens 51 is thus held in the lens holding member 53 by means of the junk ring 54 and the supporting section 53a of the lend holding member 53.
There is also available a method, as shown in FIG. 8, of arranging presser plates 55 at a plurality of positions in the circumferential direction with the optical axis of the lens 51 as the center, and securing the same to the lens holding member 53 with screws 56.
However, the conventional lens holding methods shown in FIGS. 7 and 8 as described above, have problems in that, because the lens holding member 53, the junk ring 54 or the presser plate 55 is in contact with the optical surface of the lens 51 under a prescribed pressure, the optical surface of the lens 51 deforms and optical performance such as aberration of the optical system including the lens 51 are deteriorated.
In addition, since the amount of elastic deformation of the lens 51 varies with the status of tightening of the junk ring 54 or the status of tightening of the screw 56 securing the presser plate 55, another problem is encountered in that optical performance of the optical system varies with the manner of assembly.
The lens 51 is often made of a glass material, and the lens holding member 53, the junk ring 54 and the presser plate 55 are made of a metal material in many cases. In this case, because of the difference in linear expansion coefficient, a change in temperature causes a difference in the amount of expansion/shrinkage resulting from the change in temperature between the lens 51 and the other members. This in turn causes a change in the amount of pressing (or the pressing force) of the junk ring 54 or the presser plate 55 against the lens 51, leading to a change in the amount of elastic deformation of the optical surface of the lens 51. This poses another problem of a change in the optical performance of the optical system caused by a change in temperature.